Production of dicobaltous oxychromate and percobaltous oxychromate

ABSTRACT

THE DISCLOSURE PERTAINES TO THE PREPARRATION OF DICOBALTOUS OXYCHROMATE AND PERCOBALTOUS OXYCHROMATES BY THE PRECIPITATION OF THESE COMPOUNDS FROM A REACTION MIXTURE CONTAINING CHROMIC ACID AND COBALTOUS HYDROXIDE IN WATER. WHEN THE PH OF SUCH A MIXTURE IS MAINTAINED WITHIN THE RANGE OF FROM ABOUT 4 TO ABOUT 7, THE DICOBALTOUS OXYCHROMATE IS PRODUCED; AT HIGHER PH THE PERCOBALTOUS OXYCHROMATES ARE PRODUCE. THESE COMPOUNDS ARE VALUABLE AS CORROSION INHIBITORS.

United States Patent 3,664,808 PRODUCTION OF DICOBALTOUS OXYCHRO- MATE AND PERCOBALTOUS OXYCHROMATE Kenneth E. Weber, Los Angeles, Geraldine H. Hoch, Chatsworth, and John L. Wanamaker, Burbank, Calif., azsslilgfnors to Lockheed Aircraft Corporation, Burbank, No Drawing. Filed Mar. 30, 1970, Ser. No. 24,024 Int. Cl. C01g 37/14 U.S. CI. 23-56 4 Claims ABSTRACT OF THE DISCLOSURE The disclosure pertains to the preparation of dicobaltous oxychromate and percobaltous oxychromates by the precipitation of these compounds from a reaction mixture containing chromic acid and cobaltous hydroxide in water. When the pH of such a mixture is maintained within the range of from about 4 to about 7, the dicobaltous oxychromate is produced; at a higher pH the percobaltous oxychromates are produced. These compounds are valuable as corrosion inhibitors.

BACKGROUND OF THE INVENTION It has long been considered that cobaltous hydroxide (Co(OH would react with chromic acid to produce the neutral salt cobaltous chromate (CoCrO It has also long been considered that cobaltous salts would react with chromic acid salts so as to produce the same neutral salt. Thus, for example, the article by Pelletier et al entitled Contribution to the Study of the Precipitation of Chromates appearing in the February 1938 issue of Canadian Journal of Research, volume 16, Section B indicates that cobaltous nitrate will react with potassium chromate so as to form the neutral salt cobaltous chromate.

Investigation as to the effectiveness of cobaltous chromate as a corrosion inhibitor has led to the discovery that under certain conditions the reaction of cobalt hydroxide and chromic acid does not produce this particular salt, but will produce either the corresponding dicobaltous oxychromate (2CoO-CrO or corresponding percobaltous oxychromates (xCoO-CrO in which x is greater than 2 depending on the control of the reaction conditions. This has been considered as quite surprising in view of the established literature pertaining to the production of cobalt salts. This discovery is considered to be particularly significant in view of the related discovery that the diand percobaltous oxychromates are eifective as corrosion inhibitors over varying periods because of their unique solubility characteristics.

SUMMARY OF THE INVENTION An objective of the present invention is to teach a process for the production of dicobaltous oxychromate and of percobaltous oxychromates. A further objective of this invention is to provide a process for the production of these compounds which may be carried out with a minimum of difiiculty at a comparatively nominal cost. A somewhat related objective to this invention is to provide the compound percobaltous oxychromates as expressed by the formula xCoO-CrO in which x is greater than 2.

In accordance with this invention, the compounds indicated are produced by reacting cobaltous hydroxide with chromic acid by adding this hydroxide to the acid under such conditions that the pH of the reaction solution is carefully controlled. If the pH of such a solution is within the range of from about 4 to about 7 and preferably about 5, the dicobaltous oxychromate will be produced as a precipitate; if the pH of the solution is about 7 to about 11, percobaltous oxychromates will be produced as a pre- 3,664,808 Patented May 23, 1972 ice DETAILED DISCUSSION This summary is misleading in its simplicity because the formation of dicobaltous oxychromate and percobaltous oxychromates involves a number of complex factors. These factors are best explained by indicating how the process of this invention can be carried out to produce either dicobaltous oxychromate or percobaltous oxychromates.

In the production of either of these compounds in accordance with this invention a solution of chromic acid is prepared by adding chromic oxide (CrO to water. Preferably the water used should be distilled or deionized to avoid the presence of possible contaminents. Then cobaltous hydroxide is added to this acidic solution in order to raise the pH of the solution. As this is done and as the pH is increased to within the range of from about 4 to 7, dicobaltous oxychromate will be formed after a reason-able reaction period. As the pH of the solution is increased to within the range from about 7 to 11, if the chromic ion is present percobaltous oxychromates will be formed as a result of the reaction transpiring and an equilibrium with the dicobaltous oxychromate. To obtain a pH greater than about 8.35 it is necessary to add a caustic such as sodium hydroxide to the reaction solution.

When the dicobaltous oxychromate is formed as indicated in this discussion, the initial cobaltous hydroxide added to the chromic acid solution Will be subjected to extremely acidic conditions since a chromic acid solution of a concentration as preferably used will have a pH in the neighborhood of 1. As this occurs the initially reacted hydroxide will tend to react with the acid so as to produce cobaltous dichromate (CoCr O This compound has a significant degree of solubility in water. This compound will stay in solution as greater quantities of the cobaltous hydroxide are added until such time as the increasing addition of the cobaltous hydroxide raises the pH of the solution to about 4.

At this time an equilibrium will be established between the cobaltous dichromate and dicobaltous oxychromate and the dicobaltous oxychromate will start to precipitate out as a brownish precipitate. The equilibrium between these two compounds will favor the production of this precipitate as further cobaltous hydroxide is added to a reaction solution as indicated until the pH of about 5. Such precipitation will continue as further of the cobaltous hydroxide is added to the reaction solution bringing its pH up to a pH of about 7. Throughout this pH range of 4 to 7 there will be present in the reaction vessel the dicobaltous oxychromate and a number of different ions such as the cobaltous ion, the chromate ion, and some of the dichromate ions, although the latter will not be significantly apparent.

At about this pH of 7 it is considered that another equilibrium is established. If further cobaltous hydroxide is added so as to increase the pH of the solution past about 7 the dicobaltous oxychromate will continue to be precipitated, but at the same time there will be a tendency for percobaltous oxychromates to simultaneously precipitate. These percobaltous oxychromates are similar to the dicobaltous oxychromate in appearance. The formation of percobaltous chromates will, however, only occur in significant amounts if chromate ions are available in solution for the formation of this type of compound. At a pH slightly above 7, a combined precipitate of the diand percobaltous oxychromates will form. The higher the pH of the solution goes, the greater the proportion of the percobaltous oxychromate formed.

At about a pH of 8 it is considered that percobaltous oxychromates will be formed to the almost complete exclusion of the formation of any dicobaltous oxychromate. It is considered that the formation of percobaltous oxychromates will continue up to about a pH of 11. Throughout this range the composition of the percobaltous oxychromate produced will continuously change. At a pH of about 7 to 8 the x in the formula xCoO.CrO expressing percobaltous oxychromate is slightly greater than 2. As the pH of the reaction solution increases to about 11 the value of x in this formula increases significantly. Although no precise value is known it is considered that at a pH of about 11 the value of x in this formula is in the magnitude of 100. The higher this value x in the noted formula the slower a particular percobaltous oxychromate will go into solution by a complex, decomposition type mechanism.

The formation of percobaltous oxychromates is, however, not exclusive in an alkaline solution as indicated. It is believed that starting at a pH of about 7 some cobaltous oxide will form, and that the amount of this compound formed will gradually increase with an increase in the pH until after a pH of 11 is reached. At this point, it is believed that this cobaltous oxide is formed nearly exclusively to the exclusion of the formation of the percobaltous oxychromates.

From a careful consideration of this discussion it will be realized that the formation of the dicobaltous oxychromate and the cobaltous oxide involves continuous variable situations which are dependent upon the amount of chromic acid introduced into the reaction mixture. Because of the equilibrium in the formation of dicobaltous oxychromate and percobaltous oxychromates it is considered that some small trace amounts of percobaltous oxychromates may be formed at pH values slightly less than 7, and that the amount of dicobaltous oxychromate formed above a pH of about 8 is too small to be considered. Also it is noted that the formation of the coboltous oxide gradually increases in a range which overlaps the pH range where diand percobaltous oxychromates are formed.

As a consequence of this, a separation of dicobaltous oxychromate requires careful control of the pH of the reaction solution so as to recover the precipitate formed before any significant amount of percobaltous oxychromates can be for-med. It is considered that in the production of the dicobaltous oxychromate compound that the addition of cobaltous hydroxide should be stopped at about a pH of in order to obtain a maximum recovery of the dicobaltous chromate.

A separation of a percobaltous oxychromate composition having particular solubility characteristics can be achieved by separating the precipitate which forms at a pH less than the pH at which this composition forms and then increasing the pH of the reaction solution to the pH at which this particular composition forms. As noted this increase in pH is achieved normally by the addition of cobaltous hydroxide, but at higher pHs require also the addition of a caustic.

In the production of either dicobaltous oxychromate or percobaltous oxychromates in the manner indicated, the solution remaining after the separation of a precipitate is formed will contain both cobalt and chromium in different ionic forms in solution. An important aspect of the present invention from an economic standpoint lies in fact that additional chromic acid can be added to such a recovered reaction solution, and that the process of pro ducing either dicobaltous oxychromate or percobaltous oxychromates from such a solution may be carried out by the addition of first chromic acid (CrO and then cobaltous hydroxide in an amount sufficient to bring the pH of such a reused process solution up to a pH necessary to produce the desired product.

In either reusing a process solution as described in making what amounts to a new process solution of chromic acid and cobaltous chromate it is considered important to regulate the total amount of cobalt and chromium compounds present in the reaction vessel so that an amount not over about 350 grams of material is present per liter of reaction solution. When high concentrations of material are present in the reaction vessel used in the production of cobaltous oxychromates as described there is tendency for a gel to form in the reaction vessel. It is to be emphasized however, that such a gel does not always form. The precise character of the gel which has been encountered is not known or understood. It is probably a mixture of a variety of different cobalt and chromium complexes.

The dicobaltous oxychromate which is produced as described is more soluble in aqueous solutions than percobaltous oxychromates produced as described. All of these compounds or compositions are considered to go into solution by a complex mechanism involving partial decomposition. The differentials in solubilities noted are considered to make each of the different compositions herein indicated somewhat more valuable than others in various specific corrosion prevention applications in which the solubility of the compounds has an effect on the dura tion and type of corrosion prevention obtained.

Because the dicobaltous oxychromate will go into solution faster than percobaltous oxychromates, both the diand percobaltous oxychromates may conveniently be utilized together in corrosion prevention. In this case the dicobaltous oxychromate will in effect provide shorter term protection than the percobaltous oxychromates. By using a mixture of these compounds together as an inhibitor it is thus possible to obtain comparatively intermediate and long-term corrosion protection.

In order to facilitate an understanding of this invention the following examples are set forth in this specification. It is to be understood that these examples are given for this purpose only. Other examples of a related character could be set forth in order to facilitate an understanding of this invention.

EXAMPLE 1 A solution of chromic acid can be prepared by adding to a liter of distilled water 241 grams of chromic oxide (CrO With the solution at room temperature (72 F.) 259 grams of cobalt hydroxide can be added to the solution as it is stirred. This stirring should be continued until after the end of about an hour all traces of the pink color of the cobalt ion are missing and a brownish precipitate has formed at the bottom of the reaction vessel. At this point the pH of the solution will be about 5. The precipitate produced in this manner will consist essentially of dicobaltous oxychromate. This precipitate can be separated from the process solution used, separately washed, dried and used.

EXAMPLE 2 Cobaltous hydroxide can be added to the process solution separated in Example 1 so as to bring the pH of the solution up to about 8 While maintaining this solution at room temperature and stirring it. After about an hour the precipitate formed will consist of a percobaltous oxychromate with minor or trace amounts of dicobaltous oxychromate and cobaltous oxide. This precipitate can be separated from the process solution and separately washed, dried and used.

EXAMPLE 3 Cobaltous hydroxide can be added to the process solution separated in Example 1 so as to bring the pH of the solution up to about 8 while maintaining this solution at room temperature and stirring. The precipitate formed after about an hour consisting of diand percobaltous oxychromates with a trace or minor amount of cobaltous oxide may be separated from the process solution. This product can be washed, dried and used.

Further cobaltous hydroxide can be added to the process solution resulting from this last separation to bring the pH of the solution to 8.35 While maintaining the temperature of the solution at room temperature and while agitating the solution. Over about an hour this will result in the formation in the further precipitate consisting largely of a percobaltous oxychromate and some trace of minor amounts of cobaltous oxide.

EXAMPLE 4 Sodium hydroxide can be added with the further cobaltous hydroxide in the preceding Example 3 in an amount in excess over the amount of cobaltous hydroxide necessary to bring the pH of the solution to 8.35 to bring the pH of the solution in this example to 9. The precipitate forming in about an hour will consist largely of a percobaltous oxychromate and some trace of minor amounts of cobaltous oxide.

EXAMPLE 5 Chromic acid may be added to the separated process solution in Example 1 to bring the concentration of chromic acid in the solution up to the initial concentration specified in this example. Then the quantity of cobalt hydroxide specified in Example 1 can be added to the re-acidified solution while the solution is held at room temperature and stirred. At the end of about an hour a precipitate consisting essentially of dicobaltous oxychromate will be formed and may be separated as in Example 1.

EXAMPLE 6 A combined precipitate of dicobaltous oxychromate and a percobaltous oxychromate can be prepared by slowly adding 350 grams of cobalt hydroxide to the chromic acid solution specified in Example 1 instead of the 259 grams specified in this Example 1. Otherwise the steps of this Example 6 are the same as in Example 1.

We claim: 1. A process of forming a diacobaltous oxychromate composition which comprises:

adding cobaltous hydroxide to an aqueous solution of chromic acid in an amount sufficient to bring the pH of said chromic acid solution to within the range of 4-7 so as to cause as the consequence of the addition of said cobaltous hydroxide and the pH of said solution a reaction between said cobaltous hydroxide and said chromic acid forming a dicobaltous oxychromate precipitate, and separating said precipitate from the process solution remaining after the formation of said precipitate. 2. A process of forming a composition of matter consisting essentially of dicaboltous oxychromate and percobaltous oxychromate having the formula xCoO.CrO in which x is greater than 2 which comprises:

adding cobaltous hydroxide to an aqueous solution of chromic acid in an amount of sufficient to bring the 6 pH of said chromic acid solution to within the range of from about 7 to 8 as the result of the addition of said cobaltous hydroxide so as to cause as a consequence of the addition of said cobaltous hydroxide and the pH of said solution a reaction between said cobaltous hydroxide and said chromic acid resulting in the production of a precipitate of dicobaltous oxychromate and percobaltous oxychromate having the formula xCoO.CrO in which x is greater than 2, and separating said precipitate from the process solution remaining after the formation of said precipitate.

3. A process of forming a percobaltous oxychromate composition having the formula xCoO.CrO in which x is Within the range of from 2 to which comprises:

adding cobaltous hydroxide and an inorganic hydroxide incapable of interferring with the reaction between cobaltous hydroxide and chromic acid to an aqueous solution of chromic acid in an amount sufficient to bring the pH of the said solution to within the range of from about 8 to 11 so as to cause as the consequence of the addition of said cobaltous hydroxide and the pH of said solution a reaction between said cobaltous hydroxide and said chromic acid forming a percobaltous oxychromate precipitate having the formula xCoO.CrO in which x is within the range of from 2. to 100, and

separating said precipitate from the process solution remaining after the formation of said precipitate.

4. In a process for forming a cobaltous oxychromate composition by adding to a chromic acid solution cobaltous hydroxide, such addition of cobaltous hydroxide serving to raise the pH of such solution and to cause the formation of a cobaltous oxychromate precipitate, said solution after the formation of such precipitate containing cobalt and chromate ions, the step which includes:

utilizing such solution after the separation of said precipitate for the production of further cobaltous oxychromate by the addition of chromic acid to said process solution and then by the subsequent addition of cobaltous hydroxide.

References Cited UNITED STATES PATENTS 2,060,192 11/1936 Gilbert 2356 X 2,400,272 5/ 1946 Todd 2356 2,573,738 11/1951 Smith et a1 2356 X 2,699,419 1/ 1955 Whaley et al. 2356 X OTHER REFERENCES Chemical Abstracts, volume 32, column 40 96 JULIUS FROME, Primary Examiner D. A. JACKSON, Assistant Examiner US. Cl. X.R. 106-14 

